Listeria monocytogenes is able to spread from cell-to-cell by utilizing the host-cell's mechanism of actin polymerization to propel it through the cytoplasm. Despite years of extensive investigation, this mechanism is still poorly understood, and the roles of several actin regulatory proteins have yet to be established. The function of one of these proteins, profilin, remains controversial. We propose that profilin plays a critical role in actin-based motility in Listeria and must be concentrated on the surface of the bacterium to support the rapid rates of actin assembly associated with the migration of Listeria through the host-cell cytoplasm. We believe that the profilin-actin complex is the preferred species for addition of actin monomers to the growing actin filament. To that end, we will study the function of this complex utilizing two experimental approaches. In the first approach, we will introduce profilin, actin, profilin-actin complex, or mutated profilin-actin complex into PtK2 cells infected with Listeria. We expect than the addition of the profilin-actin complex will enhance bacterial motility more than the other additions. To complement these experiments, we will also introduce the same proteins into profilin-depleted Xenopus egg extracts supporting Listeria. In cell-free model, we also expect the profilin-actin complex to be the most effective component restoring bacterial motility. Finally, we will explore the interaction of profilin with other proteins found in the actin polymerization zone using fluorescent energy transfer. These investigations are central in determining the role of profilin in actin- based motility of Listeria, which will provide further insight into how this bacteria causes disease.